The cytotoxic effect of substances upon cells in culture is measured by a cell survival/viability assay, the Neutral Red Uptake assay (NRU), which is based on the ability of the cells to incorporate and bind the Neutral Red (NR). The in vitro data are used to estimate an IC50 value which in turn is used to predict a LD50 value that can serve as the starting dose for the acute oral toxicity test in vivo. (NICEATM/ECVAM validation study protocol, method evaluated in the EU Intergrated Project "ACuteTox").